1. Field of the Invention
This invention relates generally to automatic transmissions for automobiles and more particularly to an automatic transmission combined with an internal combustion engine for vehicles.
2. Description of the Prior Art
An automatic transmission usually comprises a fluid torque converter operatively connected to an internal combustion engine for transmitting torque from the engine to the driving wheels. In cooperation of the torque converter and gear mechanism, the torque generated by the engine is smoothly transmitted in various gear ratios to the driving wheels. However, an automobile with an automatic transmission is said to be less powerful (especially at a standing start) and less fuel efficient compared with an automobile with a manual transmission. This is mainly because the capacity or the efficiency range of the engine is restricted when it is used in combination with the torque converter.
FIG. 1 shows the engine torque curve and torque converter efficiency range wherein lines X and Y indicate the torque converter efficiency range and line Z indicates the available maximum engine torque curve according to its capacity. Without the torque converter, engine torque is available uniformly below curve Z. However, if it is used in combination with the torque converter, the available range is limited to within the area enclosed by the lines X, Y and curve Z. Curve Z indicates the wide open engine throttle condition which means the engine throttle valve is fully open.
The point A is known as "stall point" which shows the condition of the vehicle wherein the brake pedal is depressed with the gear shift range in "Drive" and wherein the engine throttle valve is wide open. From this condition the time of standing start to a quarter mile is measured.
In FIG. 1 the following terms are applicable:
T.sub.E : engine torque PA1 N.sub.E : engine RPM ##EQU1## T.sub.I : impeller input torque N.sub.I : impeller input RPM
It should also be noted that WOT (original) as shown in FIG. 1 designates a wide open throttle degree.
The fact that an automobile having an automatic transmission has a longer standing start time than one having a manual transmission is based on the following reasoning. In FIG. 1, at stall point A, the horse power valve is only 40 HP although the engine is designed to provide more than 100 HP.
It may be possible for a driver to manually bring the stall point up to the point C (where the horse power is 65 HP) if the vehicle has a manual transmission. It is also possible to use another torque converter (i.e. a more efficient one) so as to raise the stall point. However, in this case the fuel efficiency will be enormously decreased due to "non-matching" of the engine and torque converter capacities. If a less efficient torque converter is used for improving the fuel efficiency, the performance as a whole may decrease due to the same above.
According to the present invention, a gear reduction of continuously variable transmission device is provided between the engine output and torque converter input so as to vary the efficiency range of the torque converter without changing the size of the torque converter itself. Thus, with this gear reduction or continuously variable transmission device, it may be possible to either raise the stall point to point B in FIG. 1 if desired to have the performance of a manual transmission vehicle or to maintain the stall point at A if desired so as to provide improved fuel efficiency. In the latter case, it is possible to replace the engine itself with a smaller one so as to keep the stall point at point A, thus improving the fuel efficiency.
As an example of a conventional device, U.S. Pat. No. 3,739,658 discloses an automatic transmission including a CVT (toric roller type) between the engine and torque converter. However, the engine specifically mentioned in this patent is a gas turbine type which is quite different from the internal combustion type. More particulary the engine power as a function of engine speed of a gas turbine type represents only a line shown on page 34 in FIG. 3.9 of the publication entitled Conceptual Design Study of Improved Automotive Gas Turbine Power Train--Final Report--National Technical Information Service (report by DOE/NASA), pages 30 through 34, published May, 1979 versus the area that the internal combustion engine has as shown in FIG. 1 (i.e. the area below curve Z).